The present invention relates generally to methods of forming coated hydrophobic foams and other cellular materials, to articles of manufacture produced from those coated foams and other cellular materials, and to methods of production thereof and relates more particularly to coated poly(4-methyl-1-pentene), to articles of manufacture produced from that foam, and to methods of producing the coated foam and the articles of manufacture. This invention is a result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Closed cell foams have in the past been used for various purposes (e.g. as insulation); and polymer coatings have been applied thereto; but such coating has not been done for the purpose of smoothing the surface of the foam.
Targets used for inertial confinement fusion (i.e., ICF) studies often are in the shape of spheres or cylinders and are hollow. They may either be made of glass or metal, but they must be strong enough to contain a gaseous fuel at considerable pressure, be free from pinholes which would allow the fuel to leak out, and be uniform on a macroscopic and microscopic scale to ensure a symmetrical and efficient implosion when they are detonated.
ICF targets have generally been made in one of two ways. In one way, they have been blown by causing a gaseous expansion within a molten material and then solidifying the hollow sphere; or in a second way, they can be produced on a shaped mandrel. In the blowing process, the target material has been confined to glass and a few other materials; and the blown targets are limited in shape to spheres or substantially sphere-shaped structures. On the other hand, using the second method, targets can be produced in a wide variety of shapes from a wide range of materials by depositing the wall material on a preformed mandrel and then dissolving out the mandrel. The deposition can be carried out, for example, by electroplating, physical vapor deposition (PVD), or chemical vapor deposition (CVD) techniques. However, removal of the mandrel has generally been a tedious and time-consuming process, especially when the mandrel consisted of a metal to be dissolved away through a small hole in an overcoated shell. Since polymers are soluble in many solvents which do not attack the usual ICF target shell materials, particularly metals, polymers would be ideal materials for use as mandrels if problems arising from their tendency to swell before they dissolve in the solvent could be overcome.
A low-density microcellular foam of poly(4-methyl-1-pentene), referred to as TPX, was produced and was the subject of Los Alamos report LA-UR-81-2722, "Preparation of Multishell ICF Target Plastic Foam Cushion Materials by Thermally Induced Phase Inversion Processes," A. T. Young et al. which also was published as J. Vac. Sci. Technol., 20(4), April 1982, pages 1094-1097, that foam is also the subject of U.S. Pat. No. 4,430,451 to Ainslie T. Young et al., wherein the foam is described as being particularly useful for forming targets for inertial confinement fusion. Articles made from the foam were machined to tolerances of 0.0001 inch, although the densities of the fragile foams are low (about 10 to about 100 mg/cc) and the cell sizes are small (about 10 to about 30 .mu.m). Articles made from the foam can be machined if the reinforcing material has not been removed. After machining, the reinforcing material can be removed by leaching the article in a liquid which dissolves the reinforcing material but not the TPX. As a result of the leaching process, an open-cell foam is produced. If the reinforcing material is removed at this step and if then the foam is coated with metal, the metal surface will have irregularities due to the open cells. On the other hand, if the reinforcing material is left within the foam during the metal coating step (particularly if that step involves use of a vacuum), it is almost certain that the resulting outgassing of the reinforcing material would prevent formation of a continuous metal coating or would fracture any metal coating that might form. It is possible that leaving the material inside would even cause the structure to disintegrate.
Therefore, despite what has been known in the prior art, a need existed until now for a method for producing a very smooth layer of metal on a cellular structure having a chosen arbitrary shape and being useful, for example, as an ICF target.